Axis History Forum

This is an apolitical forum for discussions on the Axis nations and related topics hosted by the Axis History Factbook in cooperation with Christian Ankerstjerne’s Panzerworld and Christoph Awender's WW2 day by day.
Founded in 1999.

Today I've had an idea: the 37mm US gun attacking a 50mm plate is quite similar in scale to the 76mm gun attacking a 100mm plate on Tiger I. And we know that the latter had serious troubles getting through at even small obliquities like 20-30°, because the shells were breaking up.

So, I found this report on testing of M62 APC shells against high hardness armour and chose these entries:
roughly 312BHN plate(closest to the 320BHN nominal hardness of PzKfw.III front plates):

roughly 363BHN:

Using DeMarre to estimate the ballistic limits for the M51 APC against 50mm/20° I get 781m/s and 757m/s from the first and second case respectively.
These striking velocities are approximately equivalent to 350 and 500m, pretty close to 500-600yrds. given in this table:

The 3 inches M62 APC shattering under as little as 20 deg obliquity represents a somewhat serious deficit in AP performance.
However, other US sources state that the M61 was good to 25 deg obliquity, presumably against softer, more regular US homogeneous armor, so it’s in the right ballpark, nevertheless.

The 3 inches M62 APC shattering under as little as 20 deg obliquity represents a somewhat serious deficit in AP performance.
However, other US sources state that the M61 was good to 25 deg obliquity, presumably against softer, more regular US homogeneous armor, so it’s in the right ballpark, nevertheless.

Its likely that the data on M61 was obtained with service velocities for the 75mm M3 gun, as in under 2000fps which would also help the shell stay intact.

Yes quite so. Lower velocity helps the projectile.
what I dont understand is why the US Army was ok with this sort of performance envelope at all. The US Navy had procured the Mk29 3inches APC since a long time by then. In comparative trials 1945, it was found to not suffer the same shatter issue as the Army M62.

The Army model copied a major calibre, Navy sheath hardening pattern- while the Navy model kept a more conventional decremental pattern.
The Navy was correct both in large and small calibre heat treatments and the Army somewhat failed to listen?

"The results obtained are in most cases insufficient to build a complete tactical schematic of every unique armour layout present on the enemy tanks. But, the fact that the armour of german tanks is similar in ballistic resistance to domestic armour of the same type, gives the right to use additional data from the ballistic trials of domestic armour, a great body of which was collected during the many years of manufacturing and testing. This allows to improve our estimates of the level of protection of enemy tanks.

For computing [of the tactical diagrams] the ballistic resistance of the german RHA was taken as equal to that of the domestic high hardness armour for thicknesses up to and including 40mm and to domestic medium hardness armour for 50mm plates.

Resistance of the layered(shielded) armour was takes as equivalent to that of a single plate of the same thickness.

I suppose soviet HHA was somewhat more resistent vs soviet domestic shot than german RHA, as it utilizes a more complete break up / shatter of the attacking projectile at low obliquities.

"layered" armor can mean different things. If laminated is supposedly meant, then its generous to the attacked plate, as the netto resistence of welded and/ or rivetted plates is somewhat lower. Only forged laminates delete any weakness (e.g. TIGER Ausf. E/H´s twin 40mm forged together turret side armor plates).

Spaced armor with air gap in between can be very, very unpredictable here. If the projectile breaks up on the skirting plates, the plate behind will have vastly superior resistence. The same is true for capped AP, getting stripped from nose covers. If the projectile just tumbles, the plate behind will have somewhat elevated resistence. Then, of course, the fuse can be activated early.

I suppose soviet HHA was somewhat more resistent vs domestic shot than german RHA, as it utilized a more complete break up / shatter of the attacking projectile.

"layered" armor can mean different things. If laminated is supposedly meant, then its generous to the attacked plate, as the nettor resistence of welded and/ or rivetted plates is somewhat lower. Only forged laminates delete any weakness (e.g. TIGER 1´s 2x40mm forged together, turret side armor plating).

Spaced armor with air gap in between can be very, very unpredictable here. If the projectile breaks up on the skirting plates, the plate behind will have vastly superior resistence. The same is true for capped AP, getting stripped from nose covers. If the projectile just tumbles, the plate behind will have somewhat elevated resistence. Then, of course, the fuse can be activated early.

There were no tanks with spaced armour examined here, the only instance in this document where this rule was applied(as far as I've seen) was for the 20+20mm side armour on the Pz.IV tank. And yes the writes were aware that this would likely provide less resistance than a single 40mm plate, but decided to be safe in their estimates.

If that armor was *calcuated* based upon resistence aequivalence to soviet HHA and RHA, then the dangerous range enevlope at 0° would be generally underestimated, while at high obliquities, the calculated dangerous distance would be overestimated with the crossover ranging between 30 and 35°.
But then again, this refers only to RHA but in the time in question, some plating (particularely over the fwd arc) were not RHA but FHA, with differing properties.

Thats not a critique. In absence of exact understanding of the respective obliquity differences, which were only worked out in 1945, one has make presumptions. And its so logical, one bases them on data which one understands well enough.

Things could've changed a lot in 2 years. Further developments of 8S steel in response to the need to simplify production and possible alloying element shortages during some time periods could've changed these values drastically(probably for the worse). Thankfully the document had this helpful table in it:

Yes. That helps a lot. It also directly associates to the test projectile specimen in question (acc. to drawing 2-05013). Considering the rather wide range of different ballistic results just by substituting another 45mm test projectile, this carries significance.